1. Field of the Invention
The invention relates to a pixel and a switch element thereof, and more particularly to a pixel and a switch element thereof in a liquid crystal display panel.
2. Description of the Related Art
Given requirements for moving image quality, display devices driven by high frequency signals have been developed, such as a liquid crystal display (LCD) devices with a driving frequency of 120 Hz. For high frequency LCD devices, two issues for LCD design must be considered, one is RC delay time of gate lines, and the other is charging capability of thin film transistors (TFTs) within pixels. Problems relating to RC delay time of gate lines are solved by forming gate lines from low impedance metal materials. As for charging capability of TFTs within pixels, the channel width of the TFTs within the pixels is increased to enhance the charging capability of TFTs.
FIG. 1 shows a conventional thin film transistor of a pixel in a driving frequency of 60 Hz. A source electrode SE is electrically connected to a pixel electrode PE. A drain electrode DE is electrically connected to a data line DL. A gate electrode GE is electrically connected to a scan line SL. An amorphous semiconductor layer AS is disposed between the source and drain electrodes SE and DE and the gate electrode GE. The scan line SL comprises the gate electrode GE, in other words, the gate electrode GE is provided by a part of the scan line SL. FIG. 2 shows voltage VPE of the pixel electrode PE, voltage VDL of the data line DL, and voltage VSL of the scan line SL of the pixel of FIG. 1 in a driving frequency of 60 Hz. According to FIG. 2, during a predetermined charging time TCH2, the voltage VPE of the pixel electrode PE rises and is almost equal to the voltage VDL. The charging capability of the TFT is approximately equal to 99%. FIG. 3 shows voltage VPE of the pixel electrode PE, voltage VDL of the data line DL, and voltage VSL of the scan line SL of the pixel of FIG. 1 in a driving frequency of 120 Hz. Referring to FIG. 3, since the charging time TCH3 is shorter than the charging time TCH2, the charging capability of the TFT is greatly degraded.
FIG. 4 shows one conventional TFT with an increased channel width. Referring to FIG. 4, area of the gate electrode GE, the drain electrode DE, and the amorphous semiconductor layer AS are increased to increase the channel width of the TFT. Thus, the charging capability of the TFT is enhanced to 90% in driving frequency of 120 Hz. However, a gate-source capacitor (Cgs) and a gate-drain capacitor (Cgd) are also increased, resulting in increase of RC delay time.
FIG. 5 shows another conventional TFT with an increased channel width. Referring to FIG. 5, a drain electrode is formed by “UU” form to increase channel width. The channel width is increased, and a gate-source capacitor (Cgs) is decreased. However, a gate-drain capacitor (Cgd) is greatly increased, resulting in increase of RC delay time. Moreover, the TFT with the “UU” form occupies a large area, and aperture ratio of pixels is decreased.